Impact Of Cascade Hydropower Development On River Ecosystem Health In Lancang River

The Lancang-Mekong River as the largest international river in Asia,basin in China is located in the southwestern alpine valley.In recent years,16 cascade hydropower stations have been planned in the Lancang River Basin in Yunnan Province,which makes the original river state form a group of deep-water and super-long reservoirs.This project relied on the National Natural Fund’s major research project"The impact of cascade hydropower development on biogenic substance in Lancang River Basin".Ecological survey of Lancang River Basin in February 2017,June 2017and January 2018,the basic law of water body indexes,diversity of planktonic bacteria and changes of community structure in Lancang River Basin were analyzed,the health status of the Lancang River aquatic ecosystem was evaluated and analyzed by using the Entropy Weight Comprehensive Health Index and the bacterioplankton index of biotic integrity.The main conclusions were as follows:（1）The variation range of TN concentration in Lancang River watershed ranged from 0.224 to 1.765 mg·L^-1,with an average value of 0.671 mg·L^-1,and the mass concentration of TN in different types of water body was manifested as tributary section>reservoir section>natural river section.TP concentration ranged from 0.007to 0.981 mg·L^-1,with an average value of 0.083 mg·L^-1,the concentration of TP in different types of water body was manifested as tributary section>reservoir section>Natural River section.For other factors,velocity,turbidity,dissolved oxygen,SpCond and pH of natural river section were significantly higher than those of reservoir section,while water temperature of reservoir section was significantly higher than that of natural river section.（2）Vertical distribution of water environment characteristics of each reservoir in Lancang River Basin is significantly different.GGQ,MW,DCSand JH reservoirs belong to mixed water bodies,while XW and NZD reservoirs belong to weak stratified water bodies.The vertical variations of Velocity,WT,Tur,SpCond,pH and DO of GGQ,MW,DCS and JH reservoirs are basically the same,and they are all in a stable state,while the vertical distribution of XW and NZD reservoirs varies greatly,and the difference of hydrodynamic characteristics and physical and chemical factors is more obvious.（3）The community structure of planktonic bacteria in Lancang River had high diversity.Relative abundance of Proteobacteria in the highest proportion was the main bacterial groups.The ACE index of planktonic bacteria in Lancang River Basin ranged from 2156.18 to 14361.71,with an average value of 5654.62.Shonnon index ranged from 1.16 to 5.86 with an average of 3.88.There were significant differences between ACE index and Shonnon index in natural river section and reservoir section.In terms of time,The ACE index and Shonnon index in high water season were higher than those in dry season.Spatially,the variation of ACE index and Sunnon index was higher in natural river section than in reservoir section.（4）Water temperature,dissolved oxygen,turbidity and velocity were the main driving factors for the diversity index difference of phytoplankton in Lancang River Basin;water temperature,dissolved oxygen,turbidity,spcond,velocity and nitrogen nutrients were the main factors causing the change of community structure of planktonic bacteria.（5）The average score of comprehensive health index（CHI）of The Lancang River in February 2017 and June 2017 is 0.65,which is in a healthy state.In February 2017,the average CHI of natural river section was 0.82,and that of reservoir section was 0.56.In June 2017,the average CHI of natural river section was 0.73,and that of reservoir section was 0.61.The health status of natural river section was better than that of reservoir section.（6）The BP-IBI index of Lancang River Basin ranges from 0.51 to 1.92,with an average of 1.24.In terms of time,BP-IBI index ranged from 0.97 to 1.92 in dry season,with an average value of 1.40,and from 0.51 to 1.75 in high water season,with an average value of 1.08.The health status in dry season was better than that in high water season.Spatially,the BP-IBI index of natural river season ranges from 1.41 to 1.92,with an average value of 1.71,which is at the sub-healthy level.The BP-IBI index of reservoir section ranges from 0.51 to 1.65,with an average value of 1.00,which is at the general health level.The health status of natural river section is better than that of reservoir section.